1. Field of the Invention
This invention relates generally to fuel injection for diesel engines and more particularly to a closed nozzle injector to provide different opening and closing pressures.
2. Description of the Prior Art
In fuel injectors having valve-closed nozzles, it is necessary to prevent combustion product reverse flow into the injector as a result of high cylinder pressure. A sharp end Lo the injection event can be helpful in this regard. Where a valve-closing return spring is used to close a needle valve in the injector, for example, if the spring force is high enough to close the valve against cylinder pressure to prevent admitting combustion products during combustion in a cylinder, it is higher than desirable to permit opening of the valve at a desirable nozzle opening pressure (NOP) for some operating conditions, particularly low load/idle. Negative consequences of a higher than desirable NOP include: a) a fueling curve "knee" characteristic whereby injected fueling relative to desired fueling sharply falls to zero at low load fuelings, a characteristic that leads to hot engine idle instability and cylinder power balance degradation; b) higher than desirable fuel injection rates at low idle, resulting in more pre-mixed combustion and resultant NOx formation and noise generation.
For low emissions engines, it is desirable to slow down the rate of fuel injection at the beginning of the injection event, and to make the end of injection faster or sharper. Therefore, it is desirable to provide a low NOP but a high nozzle closing pressure (NCP). U.S. Pat. No. 4,911,127 to Perr discloses an injector which has a pilot plunger 62 and a downwardly opening pilot piston 66 serving as the upper seat of a valve closing spring 86. In U.S. Pat. No. 4,605,166, a check valve 78 under control chamber 58 has a downwardly opening seat for the needle valve closing spring 80. At the end of injection, the pressure in the control chamber 58 and the bias force of spring 80 are said to exceed the lower residual pressure in the accumulator chamber 86 to rapidly reseat the valve needle.
It remains desirable to minimize wear on valve needles and seats and to minimize complexity of injectors in general, while achieving the object of a nozzle opening pressure considerably lower at the start of injection than is the closing pressure at the end of injection.